Self-restoring current limiter

ABSTRACT

The disclosed current limiters include self-restoring currentlimiting material filling one portion of a central hole and a buffer fluid filling the other portion of the hole with a piston slidably fitted filled in the hole to isolate the material from the fluid. Upon the occurrence of short-circuiting current the material evaporates to perform the current limiting operation while a very high pressure due to the evaporated material moves the piston to compress the fluid. Therefore, the pressure is buffed.

United States Patent Yamagata et al.

[54] SELF-RESTORING CURRENT LIMITER [72] inventors: Kiyoshi Yamagata; Kiyoshi Matsuura, both of Fukuyama; Toshio Ito; Toshio Miyamoto; Yuichi Wada, all of Amagasaki, all of Japan [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo, Japan [22] Filed: Apr.23, 1970 121] App]. N0.: 31,301

[30] Foreign Application Priority Data Apr. 24, 1969 Japan 44/31641 Apr. 24, 1969 Japan..... 44/31642 Apr. 24, 1969 Japan..... ..44/3l643 Apr. 24, 1969 Japan ..44/31644 US. Cl .337/21, 337/118, 337/158 Field o fSearch ..337/114, 115, 116, 117, 11s, 337/21, 158; ZOO/61.05

[ 5] Feb. 22, 1972 [56] References Cited UNITED STATES PATENTS 3,389,359 6/1968 Harris ..337/l14 3,389,360 6/1968 Keenan ..337/114 3,501,730 3/1970 lto ..337/114 Primary Examiner-.1 D. Miller Assistant Examiner1-1arvey Fendelman AnomeyRobert E. Burns and Emmanuel J. Lobato [57] ABSTRACT The disclosed current limiters include self-restoring currentlimiting material filling one portion of a central hole and a buffer fluid filling the other portion of the hole with a piston slidably fitted filled in the hole to isolate the material from the fluid. Upon the occurrence of short-circuiting current the material evaporates to perform the current limiting operation while a very high pressure due to the evaporated material moves the piston to compress the fluid. Therefore, the pressure is buffed.

6 Claims, 4 Drawing Figures BACKGROUND OF THE INVENTION This invention relates to improvements in self-restoring current limiting devices.

As well known, self-restoring type current limiting devices can repeatedly perform their current limiting operation, and, generally, such devices include a current limiting material in the form of a solid or a liquid, such material having good electrical conductivity at room temperature and being responsive to a flow of high current such as overcurrent or short-circuiting current therethrough to be evaporated through the generation of Jouls heat therein due to the flow of high current, whereby a plasma with a very high vapor pressure is formed. The evaporated material and the plasma present to the flow of short-circuiting current an electric resistance much higher than the impedance of the associated circuit thereby limiting the short-circuiting current to a value below a predetermined magnitude. After the completion of the particular currentlimiting operation, the evaporated material is rapidly cooled to room temperature to be solidified or liquidified whereupon the material is self-restored to its original good electrically conductive state to be ready for the succeeding operation.

It is commonly practiced to use any one of sodium (Na), potassium (K), alloys thereof, (NaK) etc., as the current limiting material. In any case the particular current limiting material must be evaporated in response to a flow of short-circuiting current therethrough, thus providing a large increase in vapor pressure. Therefore, an enclosure having disposed therein such a current-limiting material is subjected to undergoes a very high pressure upon evaporation of the material. This leads to the necessity of providing means for protecting the enclosure from damaging or breaking due to that very high pressure.

SUMMARY OF THE INVENTION The invention provides a current limiting device of the selfrestoring type comprising a main body having extending therethrough a central opening, a partition member fitted in sealing relationship into the opening for axial movement, sealing elements for hermetically closing each end of the opening, a predetermined quantity of current limiting material of the self-restoring type filling the opening between one of the sealing elements and the partition member, and a predetermined quantity of pressure buffer fluid filling the opening between the other sealing element and the partition member.

Therefore it is an object of the invention to provide a new and improved current limiting device of the self-restoring type, wherein the device includes provisions for preventing damage due to a very high vapor pressure occurring upon evaporating a current limiting material therein.

It is another object of the invention to prevent the abovementioned pressure buffer fluid from leaking externally of the device due to an excessive increase in pressure within that portion of the opening filled with the fluid.

It is still another object of the invention to prevent the above-mentioned partition member from being immovable during service.

It is an additional object of the invention to provide a new and improved current limiting device of the self-restoring type, which device is small in overall dimension.

BRIEF DESCRIPTION OF THE DRAWING The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawing in which:

FIG. I is a longitudinal sectional view of a current limiting device of the self-restoring type constructed in accordance with the principles of the invention with parts illustrated in plan view;

FIG. 2 is a longitudinal sectional view, partly in plan, of a modification of the invention;

FIG. 3 is a view similar to FIG. 2 but illustrating another modification of the invention; and

FIG. 4 is a view similar to FIG. 2 but illustrating still another modification of the invention.

Throughout the Figures like reference numerals designate the identical or corresponding components.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawing, it is seen that an arrangement disclosed herein comprises a first cylindrical electrode member 10 of any suitable electrically conductive material such as copper or brass having an intermediate radial shoulder and one threaded end portion 12 providing a terminal screw. The other end of the electrode 10 abuts against a cylindrical insulation 14 substantially equal in diameter thereto. The insulation 14 may be preferably of any suitable alkali resisting material such alumina, beryllia or the like. A second hollow cylindrical electrode member 16 of the same material as the first electrode member 10 firmly embraces the other end portion of the first electrode member 10 and the insulation 14 through a sleeve member 18 of any suitable electrically insulating material such as a mixture of powdered mica and powdered glass. Such a powdered mixture along with the components 10, 14 and 18 may be preferably sintered into the desired unitary structure. If desired, both insulations 14 and 18 may be of the same material.

The second electrode member 16 is radially opposed to the first electrode 10 through the sleeve-shaped insulation 18, and that end portion thereof adjacent the first electrode 10 is screw threaded into a cylindrical metallic sleeve 20, with an electrically insulating annulus 22 interposed between the shoulder on the first electrode 10 and the sleeve 20 for the purpose offirmly holding the electrodes 10 and 16in place.

As shown in FIG. 1, the one end portion 12 of the first electrode member 10 projects beyond the sleeve 20 and is provided with a threaded bore 24 communicating with a central elongated hole 26 extending through the first electrode and insulation 10 and 14, respectively, wherein the hole 26 is smaller in diameter than the threaded bore 24. That end portion of the hole 26 adjacent the bore 24 may be preferably smaller in diameter than the remaining portion thereof. Then a first sealing element or end plug 28 in the form of a needle valve is screw threaded into the bore 26 until it hermetically closes the adjacent end of the central hole 26.

That end portion of the second electrode member 16 remote from the first electrode member 10 includes a central, partly threaded opening 30 into which a cylindrical metallic buffer block 32 is screw threaded so as to project beyond the second electrode member 16. Then, an annular gasket 34 is interposed between the bottom of the central opening 30 in the second electrode 16 and the adjacent end of the buffer block 32 for sealing purposes. The block 32 also provides a terminal screw serving to connect the device to the associated circuit as does the one end portion 12 of the first electrode 10. The cylindrical block 32 has centrally extending therethrough a stepped hole composed of a threaded hole portion 36 disposed in the exposed end portion thereof and similar to the end bore 24 in the first electrode 10; an intermediate hole portion 38 smaller in diameter than the hole portion 36; and a hole portion 40 larger in diameterthan the hole portions 36 and 38 and communicating with the hole 26 extending through the first, electrode and insulation 10 and 14, and through a hole 42 centrally extending through the second electrode member 16 and having its diameter smaller than that of the hole portion 40. A second sealing element 44 also in the form of a needle valve is screw threaded into the threaded hole portion 36 to hermetically close the end hole portion 38 and therefore the larger hole portion 40.

Thus it will be appreciated that a stepped hole extends through a main device body composed of the first electrode 10, the insulation 14, the sleeve-shaped insulation 18, the second electrode 16, and the buffer block 32, and is hermetically sealed at both ends by the first and second sealing elements 28 and 44 respectively. The main body is generally designated by the reference numeral 46.

As shown in FIG. 1, a partition member 48 in the form of a piston is hermetically fitted into the hole portion 40 in the buffer block 32 for axial movement, for the purpose explained below. The partition member 48 includes an O-ring 50 slidably contacted in sealing relationship by the inner wall surface of the hole portion 40 to hermetically isolate one side from the other side thereof. A predetermined quantity of selfrestoring type current limiting material 52 fills a space formed in the composite hole 26, 42, 40 between the first sealing element 28 and the partition member 48 while a predetermined quantity of any suitable inert gas 54 such as argon under high pressure fills a space formed in the hole portions 38 and 40 between the second sealing element 44 and the partition member 48 to serve as a pressure buffer fluid.

It is well known that sodium (Na), potassium (K), alloys thereof (NaK) etc., are most suitable for use as the current limiting material 52 of the self-restoring type. Such elements and their alloys have melting points listed in the following Table I.

TABLE I Melting points of Suitable Current Limiting Materials Sodium (Na) in /c Wt. 100 56 22 Potassium (K) in 7: wt. 0 44 78 I00 Melting Point in C. 98 I9 64 Any of such metals which have relatively low melting points can readily fill the arrangement as above described. For example, sodium, having the highest melting point among the alkali metals listed in the above Table I and considered to be deficient for filling conditions can be readily filled into the device by preheating and maintaining the entire device at about l00 C. and then releasing the first sealing element 28 after which the interior thereof is evacuated to permit a predetermined quantity of liquid sodium to be poured into the holes 26 and 40 between the sealing element 28 and the partition member 48. After the pouring of the sodium has been completed the sealing element 28 is returned back to its normal position where it seals the end of the hole portion. Then the device is left to be cooled to room temperature. Subsequently the buffer fluid such as argon under a high pressure of 100 kg./mm. for example, can be added to the hole portion 40 on the right of the partition member 48 as viewed in FIG. 1 and the smaller hole portion 38 through the second or opposite sealing element 44. Thus, the sodium and argon are disposed in an opposed relation within the elongated hole in the device with partition member 48 interposed therebetween to isolate them from each other.

The filling operation as above described is not not required tobe performed at elevated temperatures and therefore does not at all damage all the insulations due to heating. In addition, the operation is safely and efficiently performed.

It will be understood that the filling operation is not limited to,the foregoing and that it may be performed in any other suitable manner. For example, sodium or potassium may be intruded under pressure into the hole through the sealing element 28 because such an element is very plastic at room temperature although it is in the form of a solid at such temperature. In any event, it is to be understood that the alkali metal itself charged in the hole is in the form of a solid, a liquid or a gas in accordance with its temperature.

If a fault current such as a shortcircuiting current flows through the device of FIG. 1 the current limiting material is at least partly evaporated to form a plasma. The evaporated material presents to the flow of shortcircuiting current a resistance higher than the impedance of the associated circuit thereby to limit the current below a predetermined magnitude. Upon evaporation of the current-limiting material, a high .vapor pressure is developed to move the partition member 48 towards the second sealing element 44 thereby to compress the fluid 54 disposed therebetween. This compression of the fluid 54 buffs the high vapor pressure ensuring that the device is prevented from damaging.

FIG. 2 shows a modification of the invention wherein the hole portion 40 in the buffer block 42, on that end near to the second sealing element 44, is provided with a recess or a fluid pocket 60 smaller in diameter than the partition member 48. In other respects the arrangement is substantially identical to that shown in FIG. 1.

Upon the occurrence of a fault current such as a short-circutiing current, the partition member 48 is moved toward the second sealing element 44, and therefore toward the fluid pocket 60 as above described in conjunction with FIG. 1. In FIG. 2, the partition member 48 abuts against the bottom sur face of the hole portion 40 to stop at the inlet of the fluid pocket 60 preventing the buffer fluid 54 from being further compressed. It is estimated that the evaporation of an alkali metal will exert upon the partition member or piston a pressure in the order of 2,000 kg./mm. that the pressure of the buffer fluid 54 will suddenly increase to at least 1,000 kg./mm. or more provided that the volume of the fluid is unlimitedly compressible in the absence of a fluid pocket 60. Under these circumstances the buffer fluid will instantaneously push the sealing element 44 aside, whereupon the fluid will leak into the atmosphere through the latter. In this connection it is noted that the current-limiting material seldom, if ever leaks through the opposite sealing element 28. This is because that portion adjacent the sealing element of the current limiting material becomes free of gas and can not leak through the sealing element.

It has been found that by properly selecting a ratio of the volume of the fluid pocket to the total volume of the buffer fluid, it is possible to control a maximum possible instantaneous pressure to an appropriate'magnitude, for example, 600 kg./mm." thereby to prevent any leakage of the buffer fluid. If the buffer fluid would leak through the sealing element 44 the piston or the partition member 48 will lack its force tending to thrust the current limiting material in its original position after the interruption of the fault current, thus leading to an increase in electric resistance and therefore a decrease in current capacity of the device. Eventually, it would be impossible to use the device.

Therefore it will be appreciated that the arrangement of FIG. 2 includes means for preventing any leakage of the buffer fluid to provide the self-restoring type of current-limiting device having a long useful life to be good for many times of the current limiting operations.

If the opposite bottom surfaces of the partition member and hole portion 48 and 40, respectively, are clean and free of foreign matter in the arrangement of FIG. 2, the bottom surface of the partition member 48 can strike against the bottom surface of the hole portion 40 under a high pressure to effect the so-called explosion junction or welding whereby the molecules of the metallic materials of both components are intertangled. Accordingly the partition member 48 would be rigidly attached to the bottom of the hole portion 40 to be incapable of returning back to its original position.

The invention also contemplates to eliminate that disadvantage by the provision of any suitable lubricant disposed between the partition member 48 and the bottom of the hole portion 40 as shown in FIG. 3. As shown, a layer of any suitable lubricant 62 such as oil, grease, molybdenum disulfide or the like it attached on the each of the bottom surfaces A and B respectively of the partition member and the hole portion 48 and 40. In other respects the arrangement is identical to that shown in FIG. 2.

FIG. 4 shows an arrangement substantially identical to that illustrated in FIG. 2 excepting that the partition member 48 is normally located at such a position that when the partition member abuts against the bottom of the hole portion upon evaporating the current limiting material, the O-ring 50 included therein contacts in sealing relationship the inner wall surface of the hole portion at its position spaced inwarded of the plane of the exposed end surface of the second electrode member 16. As shown in FIG. 4, the O-ring 50 is normally located at its position labeled C. When the partition member 48 abuts against the bottom of the hole portion 40 the O-ring 50 is moved to its position labeled C" to the left of the exposed end of the second electrode 16 as viewed in FIG. 4. That is, the moved O-ring 50 does not go over the plane of that end labeled D in the right-hand direction as viewed in Flg. 4.

This measure is effective for preventing the exposed portion of the buffer block 42 from tending to radially expand due to a high vapor pressure provided by the evaporated current limiting material.

As above described, the current limiting material responds to a flow of fault current therethrough to be at least partly evaporated to generate a vapor pressure that instantaneously exceeds 2,000 kg./mm. or more, even though the buffer fluid would exert itself to the utmost. That high pressure then tends to radially expand that portion of the block 32 disposed to the left of the moved O-ring 50 as viewed in FIG. 4. In this example, the moved O-ring 50 is still within a portion of the buffer block 32 which is firmly encircled by the main body 46 so that the block 32 is prevented from being deformed due to the high vapor pressure.

On the other hand, the buffer fluid 54 can have a pressure instantaneously amounting to several hundred kilograms per square millimeter and buffer block 32 itself can withstand such a magnitude of the pressure without relying upon any other reinforcing means.

Although there has been some fear that a pressure difference between the side of the current limiting material 52 and the side of buffer fluid 54 might break the sealing member or O-ring 50 it has been found that the use ofa good quality 0- ring as the sealing member 50 combined with improved fitting of the partition member 48 into the hole portion 40 can satisfactorily withstand the axial pressure applied to the partition member. AFter the fault current has been limited in the manner as previously described, a circuit breaker (not shown) connected in series circuit relationship to the device can be opened. Such a circuit breaker may be similarly connected to the devices as shown in each of FIGS. 1, 2 and 3. Thereafter the current limiting material in a gaseous state is cooled into a liquid or a solid state. After having returned back to its original state the current limiting material can be pressurized by the buffer fluid 52 through the partition member 48, whereby a contact resistance between the same and each of the electrode members or 16 is maintained. Therefore the device is ready for the succeeding operation.

While the invention has been illustrated and described in conjunction with a few preferred embodiments thereof it is to be understood that various changes in the details of construction and the arrangement and combination of parts may be resorted to without departing from the spirit and scope of the invention. For example, the buffer block 42 may be operatively connected to the second electrode member 16 by any suitable mean other than screw-threading means.

What we claim is:

l. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectively connected to said main body and each including a needle valve for respectively closing and hermetically sealing the opposed ends of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, and a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member.

2. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectivelyl connected to said main body forhermetically closing eac end of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member, said passageway defining a fluidcontaining pocket disposed between said other sealing element and said partition member, and means for preventing said partition member from entering the fluid-containing pocket.

3. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectively connected to said main body for hermetically closing each end of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member, and a lubricant disposed on that side of the partition member facing the buffer fluid to prevent the partition member from explosion welding to the adjacent portion of the main body.

4. A current limiting device of the self-restoring type comprising a main body having extending therethrough a first elongated passageway, a pressure buffer cylindrical block rigidly fitted into one end of the main body and having extending therethrough a second elongated passageway communicating with the first passageway, a pair of sealing elements respectively connected for hermetically closing that end of each of the passageway remote from the other passageway, a partition member fitted into the second elongated passageway for axial movement and including a sealing portion to maintain the partition member in a hermetically sealing relationship with respect to an inner wall surface of the second passageway, means for maintaining said partition member in the second elongated passageway and axially disposed between the junction of the first and second elongated passageways and said one end of the main body, a predetermined quantity of self-restoring type current limiting material filling the first and second elongated passageways between one of the sealing elements and the partition member, and a predetermined quantity of pressure buffer fluid filling the second passageway between the partition member and the other sealing element.

5. A current limiting device of the self-restoring type as claimed in claim 4 further comprising fluid pocket means disposed at the end of said second elongated passageway which is remote from said first passageway in opposite relationship with respect to the partition member, wherein said pocket means is dimensioned such that the partition member can not enter therein.

6. A current limiting device of the self-restoring type as claimed in claim 4 comprising a lubricant disposed on that side of the partition member facing the buffer fluid to prevent the partition member from explosion welding to the adjacent portion of the main body. 

1. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectively connected to said main body and each including a needle valve for respectively closing and hermetically sealing the opposed ends of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, and a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member.
 2. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectively connected to said main body for hermetically closing each end of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member, said passageway defining a fluid-containing pocket disposed between said other sealing element and said partition member, and means for preventing said partition member from entering the fluid-containing pocket.
 3. A current limiting device of the self-restoring type comprising a main body having extending therethrough an elongated passageway, a partition member fitted sealingly into the passageway for axial movement, a pair of sealing elements respectively connected to said main body for hermetically closing each end of the passageway, a predetermined quantity of current limiting material of the self-restoring type filling the passageway between one of the sealing elements and the partition member, a predetermined quantity of pressure buffer fluid filling the passageway between the other sealing element and the partition member, and a lubricant disposed on that side of the partition member facing the buffer fluid to prevent the partition member from explosion welding to the adjacent portion of the main body.
 4. A current limiting device of the self-restoring type comprising a main body having extending therethrough a first elongated passageway, a pressure buffer cylindrical block rigidly fitted into one end of the main body and having extending therethrough a second elongated passageway communicating with the first passageway, a pair of sealing elements respectively connected for hermetically closing that end of each of the passageway remote from the other passageway, a pArtition member fitted into the second elongated passageway for axial movement and including a sealing portion to maintain the partition member in a hermetically sealing relationship with respect to an inner wall surface of the second passageway, means for maintaining said partition member in the second elongated passageway and axially disposed between the junction of the first and second elongated passageways and said one end of the main body, a predetermined quantity of self-restoring type current limiting material filling the first and second elongated passageways between one of the sealing elements and the partition member, and a predetermined quantity of pressure buffer fluid filling the second passageway between the partition member and the other sealing element.
 5. A current limiting device of the self-restoring type as claimed in claim 4 further comprising fluid pocket means disposed at the end of said second elongated passageway which is remote from said first passageway in opposite relationship with respect to the partition member, wherein said pocket means is dimensioned such that the partition member can not enter therein.
 6. A current limiting device of the self-restoring type as claimed in claim 4 comprising a lubricant disposed on that side of the partition member facing the buffer fluid to prevent the partition member from explosion welding to the adjacent portion of the main body. 